Fat and oils (the term "fats and oils" as used herein means triglycerides) and fatty acid esters derived therefrom (the term "fatty acid esters" as used herein means esters of fatty acids with lower or higher alcohols other than triglycerides) usually contain at least several to several tens ppm of sulfur. In the production of alcohols starting from these fats and oils or fatty acid esters (i.e., "starting oil" or "starting ester") in the presence of an ester reduction catalyst, sulfur compounds contained in a trace amount in the starting oils would act as catalyst poisons and thus considerably shorten the activity life of the ester reduction catalyst.
Thus, the present inventors have examined various refining processes for reducing the sulfur contents of fats or fatty acid esters to be used as a starting material for producing alcohols. As a result, they have clarified the following problems.
(1) Problems in purification by distillation: PA0 (2) Problems in purification with the use of desulfurization catalysts: PA0 (3) Problems in other purification procedures (treatment with adsorbents, steaming or alkali-treatment):
When fatty acid methyl esters derived from natural fats and oils in a conventional manner are distilled, the sulfur content is reduced to 10% and 20%, based on the initial content, respectively at distillation yields of 90% and 98%. In order to reduce the sulfur content of commonly available fatty acid methyl esters to a desired level, however, a distillation loss of 5% or greater is unavoidable. In this case, further, the alkyl distribution in the material is considerably changed.
In the case of fats and oils or esters fatty acids with higher alcohols having a higher boiling point, it is practically difficult to eliminate sulfur compounds contained therein by distillation.
In the field of petroleum refining, molybdenum or tungsten catalysts have been employed for eliminating sulfur compounds contained in gas oil or oil fuel as described, for example, in Shokubai Process Kagcaku (Chemistry of Catalytic Process), published by Tokyo Kagaku Dojin.
However it is required to heat these catalysts to 300.degree. C. or above in order to achieve the desulfurized activity. When fats and oils or fatty acid esters are hydrogenated at such a high temperature, the hydrogenolysis of ester groups results in an increase in an acid value (AV) and accelerates the decomposition of materials.
In these procedures, even though fully performed, from 3 to 5 ppm of sulfur would remain and, therefore, the desired fats or fatty acid esters of a sulfur content (sulfur content: 0.6 ppm or below) cannot be obtained thereby.